Neutron-coincidence counting ("NCC") is used routinely around the world for the non-destructive mass assay of uranium and plutonium in many forms. Passive neutron multiplicity counting ("PNMC") is used routinely for the nondestructive assay ("NDA") of plutonium scrap and waste. During the fission process multiple neutrons are emitted within a very short time frame, that is, in coincidence. The number of neutrons emitted in coincidence determines the multiplicity of the event.
For nondestructive analysis the sample in question is assayed by the detection of coincident fission neutrons from the spontaneous fission of, even-mass plutonium isotopes in the presence of a random neutron background (e.g., (.alpha.,n) reactions).
As is well known, .sup.3 He neutron detectors are much more efficient for detecting slow or thermal neutrons, compared to fast neutrons. When a thermal neutron collides with a .sup.3 He molecule, a voltage pulse is produced. FIG. 1 is a schematic illustrating this process, in which 11 is a gas tube having a casing 13 and an anode wire 15. Casing 13 is filled with .sup.3 He. Anode wire 15 is connected to source of high voltage 17 (e.g., 1700 volts), a capacitor 19 and a resistance 21, as is well known in the art. The ionization resulting from a thermal neutron colliding with a .sup.3 He molecule produces a voltage pulse, as is also illustrated in FIG. 1.
Neutrons originating from (.alpha., n) reactions in the sample, from external sources, or different fissions are uncorrelated in time (i.e., random), whereas neutrons emitted by the same fissioning nucleus are time correlated. Typically, to distinguish correlated neutron events from random events (including neutrons from different fissions), two equal time periods are sampled by a coincidence circuit after a neutron has been detected. These circuits are also known as shift-register circuits. Los Alamos publication LA-UR-96-2462, "A 2-Fold Reduction in Measurement Time for Neutron Assay: Initial Tests of a Dual-Gated Shift Register (DGSR)" discloses unequal time periods, in which the R+A gate is 64 .mu.s while the A gate is 1024 .mu.s.
FIG. 2 is a neutron detection probability vs. time diagram depicting the operation of conventional shift register circuits. For a fission at time zero, the probability of detecting a fission neutron at time t decreases exponentially with time, namely: EQU P(t)=(1/.tau.) exp (-t/.tau.) (Eq. 1)
Where .tau. is the neutron "die-away time." After a long delay, .DELTA., the probability of detecting a neutron from a fission at t=0, is negligible. Therefore, upon detecting a neutron at time t, conventional shift register circuits count real coincidences R (neutron pulses from the same fission) plus accidental coincidences A (neutron pulses from other fissions plus time-random neutron pulses, e.g., from (.alpha.,n) reactions), in the time interval t+p to t+p+G, where G is the gate length, and p is the predelay. The predelay p removes bias due to electronic deadtime effects. Upon detecting a neutron at time t, conventional shift register circuits also count accidental coincidences A in the interval t+p+G+.DELTA. to t+p+2G+.DELTA. where .DELTA. represents a long delay (e.g. 1 ms). At the end of the counting interval, one quantity of interest is the number of real coincidence pairs, or doubles (D), one NDA signature for fissile material mass. For doubles, the unfolding of R from R+A is simple: EQU D=(D+A.sub.D)-(A.sub.D) (Eq. 2)
Where D is the real doubles and A.sub.D is the accidental doubles. The statistical error in D (precision) is given approximately by: EQU .sigma..sub.D =[(D+A.sub.D)+A.sub.D ].sup.1/2 (Eq. 3)
In most actual cases, D&lt;&lt;A.sub.D, and the D error is approximately EQU .sigma..sub.D =(2A.sub.D).sup.1/2 (Eq. 4)
The error model in Equation 3 is based on the assumptions of independent errors in (D+A.sub.D) and A.sub.D as well as Poisson statistics. Neither of these assumptions is valid for coincidence counting. However, this simple error model agrees with doubles sample-standard-deviation measurements to within a few tenths of a percent, depending on the item measured.
A conventional shift register circuit 111, which is illustrated in FIG. 3, includes a predelay 113, a shift register 115, an up-down counter 117, R+A accumulator (a/k/a sum) 119, A accumulator 121, R+A multiplicity accumulator 123, A multiplicity accumulator 125, and a strobe 127. In operation, a pulse entering shift register 115 increments (+1) up-down counter 117, while a pulse leaving shift register 115 decrements (-1) up-down counter 117. Thus, the number of pulses in shift register 115 is just the count in up-down counter 117. When a digital pulse 131 (a trigger pulse) crosses trigger point 133, strobe 127 is triggered. The contents of up-down counter 117 are added to R+A accumulator 119 and A accumulator 121, as well as R+A multiplicity accumulator 123 and A multiplicity accumulator 125, as indicated by strobe arrows 135, 137, 139 and 141. As those skilled in the art will appreciate, the strobe for accumulators 119 and 123 is simultaneous and occurs immediately upon a pulse crossing trigger 133, whereas the strobe for accumulators 121 and 125 which is also simultaneous is delayed by long delay 143 (e.g. 1 ms). The total number of trigger pulses is accumulated in totals register 145. Because a neutron pulse which enters the predelay 113 is produced at a later time than those neutron pulses already in shift register 115, the R+A accumulator actually tallies events which precede the neutron pulses which strobe the accumulators. This is functionally equivalent to the conceptual timing diagram of FIG. 2.
Precisions of neutron coincidence counting and neutron multiplicity counting are largely determined by the level of accidental coincidences pulses A. The higher A, the worse the precision. Thus, many neutron coincidence counting and neutron multiplicity counting assays are precision-limited, and require long count times for acceptable results. In the past, attempts to improve precision have been focused on detector design.
The article A New System for Analyzing Neutron Multiplicities: Characterization and Some Specific Applications, G. S. Brunson and G. J. Arnone, Los Alamos National Laboratories, LA-11701-MS (November, 1989), discusses decoupling the R+A and A accumulators. However, no purpose is mentioned for decoupling, and the disclosed circuit is not capable of the high sampling rates (e.g., 4 MHz) necessary for precision improvement. The circuit is limited to a pulse rate of 12.5 KHz and an A accumulator sampling rate of 125 KHz.
Accordingly, it is an object of the present invention to significantly improve neutron coincidence and neutron multiplicity counting precision by decoupling the sampling of the R+A gate from the A gate.
It is another object of the present invention to measure the R+A gate at the pulse rate, while measuring the A gate at a clock rate of 4 MHz, much faster (e.g., a factor of 5 to 10) than the pulse rate, thereby increasing the measurement precision of accidental coincidences. The greater the difference between the pulse rate and the clock rate, the greater the gain in precision of A. This, in turn, improves the precision of R, because R is obtained by unfolding R from R+A. Here, R, R+A, and A, can be either simple sums, in the case of conventional neutron coincidence counting, or pulse multiplicity distributions (0s, 1s, 2s, etc.) for neutron multiplicity counting.
It is a further object of the present invention to improve measurement precision, which permits, for a fixed precision, a significant reduction in measurement time.
It is a further object of the invention to reduce doubles measurement times by factors of 1.6 to 2.0 and to reduce passive neutron multiplicity assay times by factors of 1.7 to 2.1.
It is yet a further object to improve nuclear material assays for nonproliferation and international safeguards by the reduction of measurement times.